Proton exchange fuel cells, including polymer electrolyte membrane fuel cells, offer clean and efficient energy conversion for power generation by converting chemical energy into electrical energy. Electrochemical conversion is effected by introducing an oxygen-containing oxidant gas through a gas diffusion cathode and introducing a hydrogen-containing fuel gas through a gas diffusion anode. Protons migrate into a proton conducting electrolyte medium containing a proton conductor and react with reduced oxygen to form water. To facilitate chemical conversion, platinum containing electrodes are generally employed.
Several types of fuel cells have been developed based on the type of the fuel employed, e.g., hydrogen, natural gas, gasoline, and alcohols; their operating conditions, e.g., low or high temperature; and the type of proton conducting electrolyte used in the fuel cell. Examples of fuel cells include the alkaline fuel cell, polymeric-electrolyte-membrane (PEM) fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, and solid-oxide fuel cells.
Low temperature operation of a fuel cell (<100° C.) typically requires using very pure hydrogen as the hydrogen source. However, this source is relatively expensive and requires complex hydrogen storage devices. Hydrogen from carbon based fuels, e.g., that which comes from the water-gas-shift reactionCO+H2OCO2+H2 is less costly and easier to store. However, such fuel sources contain various amounts of carbon monoxide, and at <100° C. a 10 ppm level of carbon monoxide may poison the platinum catalyst at the cathode and anode via adsorption. Higher temperature operation of proton membrane exchange fuel cells (at least 100 to 250° C.) significantly reduces the effect of carbon monoxide adsorption. Higher temperature may also improve reaction kinetics and efficiency of the fuel cell.
Higher temperature operation of fuel cells creates significant challenges in fuel cell design. One of these challenges is in the selection of the proton conducting electrolyte. Proton conducting media employed in fuel cells operating at high temperature should have one or more of the following: high conductivity, good chemical, electrochemical, and morphological stability, oxidation resistance, good hydrogen and oxygen solubility, and an optimal interaction with the electrode catalyst material(s).
The hydrated perfluorinated sulfonic acid polymer, such as, Nafion™ (a trade mark of DuPont), and similar materials that are also commercially available, are commonly used as proton conducting electrolytes for low temperature fuel cell operation. However, because the proton-conduction mechanism in Nafion™-type membranes is based on the migration of hydrated protons, fuel cells using Nafion™-type membranes require a complicated water management system and pressurized operation above 100° C. Phosphoric acid cells offer an opportunity to operate fuel cells at high temperature but the phosphate anions are strongly absorbed on the platinum catalyst. A proton conducting electrolyte having strong adsorption characteristics in the platinum catalyst results in a loss of active sites for oxygen reduction, and a correspondingly low current density, lowering the power density of the fuel cell.
The following patents and articles are representative of the state of the art with respect to proton conducting membranes for use in fuel cells and electrochemical devices.
U.S. Pat. No. 6,468,684, discloses solid acid electrolytes of the general formula MaHb(XOt)c where H is a proton, M is a metal such as Li, Be, Na, and Mg, X is Si, P, S, As and a, b, c, and t are rational numbers, for use as proton conducting materials. These electrolytes do not require hydration and can be operated at temperatures above 100° C. Composite membranes fabricated from the solid acid, CsHSO4, a representative of this class show conductivities as high as 8 mS cm−1 at 146° C. in humidified air (pH2O=3.13×10−2 atm).
U.S. Pat. No. 5,344,722 discloses a phosphoric acid fuel cell in which the electrolyte includes phosphoric acid and a fluorinated compound, such as a salt of nonafluorobutanesulphonate or a silicone compound such as polyalkylsiloxane, e.g., polymethylsiloxane.
It is reported in Surface Electrochemistry J. O. M. Bockris and S. U. M. Khan, Plenum Press, p 887 that aqueous solutions of trifluoromethanesulfonic acid show a higher oxygen reduction rate on a platinum catalyst than solutions of phosphoric acid, presumably because of an improved oxygen solubility in the electrolyte and a lower adsorption of the acid at the Pt catalyst surface.
Alberti, et al in the article entitled, Solid State Protonic Conductors, Present Main Application and Future Prospects, Solid State Ionics, 145 (2001) 3-16 disclose a wide variety of proton conducting membranes for fuel cells. Examples of proton conducting materials include proton-conducting polymers impregnated with hydrophilic additives, such as heteropolyacids, zirconium phosphate, sulfated zirconia; sulfonated polyether ketones; and solid acid electrolytes, such as perfluorinated sulfonic acid polymers.
Yang, et al, in the article, Approaches And Technical Challenges To High Temperature Operation Of Proton Exchange Membrane Fuel Cells, Journal of Power Sources, 103, (2001), 1-9 disclose fuel cells employing a platinum anode catalyst. Composite membranes based upon perfluorinated sulfonic acids (Nafion™) and zirconium hydrogen phosphate as well as imidazole/Nafion membranes are mentioned.
U.S. Pat. No. 6,059,943 discloses solid-state, inorganic-organic composite membranes useful as ionically conducting membranes in electrochemical devices. Examples are based upon oxidation resistant polymeric matrices filled with inorganic oxide particles. Organic polymers include polytetrafluoroethylene, perfluorosulfonic acid, polysulfones and the like, while inorganic oxides are based upon heteropolytungstates, heteropolymolybdates, anions of tantalum and niobium, etc.
Rupich, et al in the article entitled Characterization of Chloroclosoborane Acids as Electrolytes for Acid Fuel Cells, J. Electrochem. Soc. 1985, 132, 119 disclose hydrated chloroclosoborane acids, H2B10Cl10 and H2B12Cl12 as alternative liquid electrolytes for intermediate temperature fuel cells. As described in this reference, aqueous solutions of these acids also show poor oxidative stability and a stronger adsorption on the Pt catalyst than aqueous solutions of sulfuric acid which itself adsorbs strongly on the Pt cathode.
All the patents cited above are incorporated herein by reference.